1. Field of the Invention
The present invention relates to a press belt that is used in such industries as papermaking, magnetic-recording-medium manufacturing and textile for applying pressure to workpieces to be pressed, with reduced occurrence of pinholes, and to a method of manufacturing the press belt.
2. Description of the Background Art
In various types of industries, a belt press process is generally used in which a continuous workpiece to be pressed is placed on a press belt and the workpiece is pressurized between one pressure member positioned inside the periphery of the press belt and the other pressure member positioned outside the periphery of the press belt. As the pressure members, a press roll and a pressure shoe for example are used.
One form of the belt press is for example shoe press that is employed as dewatering press in the papermaking industry. With the aforementioned shoe press, in order to improve the efficiency of dewatering wet paper, to a workpiece to be pressed (wet paper) placed on the outer peripheral surface of a press belt, plane pressure is applied for pressurizing (dewatering) the workpiece via the press belt between a press roll used as external pressure means positioned outside the periphery of the press belt and a pressure shoe used as internal pressure means positioned inside the periphery of the press belt. The pressure shoe used has a predetermined width in the running direction (of the workpiece to be pressed), and thus the nip width can be increased to improve dewatering efficiency.
A press belt has generally been used that is made of a reinforcing base material and thermosetting polyurethane integrated and shaped into an endless belt. As the reinforcing base material, a woven fabric is commonly used that is constructed of multifilaments and monofilaments, the multifilaments each being a bundle of many filaments of small fineness for adding strength.
An elastic belt for papermaking is manufactured by providing an elastic material cast on the surface of a reinforcing base material. Generally, in the process of manufacturing, a resin layer to constitute the inner surface is formed, the base material is reversed and thereafter a resin layer to constitute the outer surface is formed. Multifilaments are apt to hold air therein because of its construction. Therefore, in the process of forming the resin layer to constitute the outer surface, if resin of the outer surface is cast while the multifilaments still hold air therein, the air in the multifilaments can escape nowhere to enter the resin of the outer surface. Air babbles flowing into the resin layer which is cured to a certain extent cannot flow out of the resin layer and thus remain in the resin layer. Accordingly, the resultant product, namely press belt has pinholes left therein. The presence of the pinholes deteriorates the external appearance and causes such a problem as the one that cracks originate from the pinholes.
Japanese Patent Laying-Open No. 4-119191 for example proposes a method, with the purpose of preventing occurrence of pinholes, to form an intermediate elastic layer on at least one surface of a base fabric layer and form a surface elastic layer on the outside of the intermediate elastic layer. This method can considerably reduce occurrence of pinholes since it can expel, in the process of forming the intermediate layer, air in gaps between constituent yarns of the base fabric layer. However, it has been difficult, when a base fabric containing multifilaments is used, to prevent air babbles flowing out of the multifilaments.
Further, Japanese Patent Laying-Open No. 2002-146694 discloses a belt for papermaking that is structured by burying a reinforcing base material in thermosetting polyurethane, and proposes a method according to which polyurethane that constitutes the outer peripheral surface is made of a composite of a urethane polymer having isocyanate terminal groups and a curing agent containing dimethylthiotoluenediamine.
Using this method, a belt for papermaking can be produced while hindering growth of opened cracks and reducing occurrence of interlayer peeling between the reinforcing base material and the polyurethane.
Regarding the above-discussed methods of Japanese Patent Laying-Open Nos. 4-119191 and 2002-146694, however, the effect of reducing occurrence of pinholes due to gaps in multifilaments can further be improved.